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Among parasitic platyhelminths with complex life cycles, it has been well documented that transmission opportunities are the main forces shaping the diversity of life-history traits and parasite developmental strategies. While deviations in the development pathway usually involve shortening of life cycles, their extension may also occur following perception of remaining time by parasites. Polystoma gallieni, the monogenean parasite of Hyla meridionalis, is able to trigger two alternative developmental strategies depending on the physiological stage of the tadpoles upon which larvae attach. The distribution and reproductive outputs of both resulting phenotypes were surveyed to address questions about the dynamics of transmission in natural environments. Because modifications in the completion of life cycles can have drawbacks which may perturb the dynamic equilibrium of the resulting host-parasite systems, experimental infestations were also performed to assess parasite-parasite interactions. Our results suggest that the bladder adult phenotype, which involves transmission between frogs and tadpoles, is supplied secondarily by the branchial phenotype which involves transmission between tadpoles and metamorphs. They also support the occurrence of finely tuned trade-offs between hosts and parasites and highlight positive trends behind the extension of direct life cycles, in which host-derived signals account for the remaining time to achieve parasitic transmission., (© 2010 The Authors. Journal Compilation © 2010 European Society For Evolutionary Biology.)

Sparicotyle chrysophrii infections were highlighted during a parasitological monitoring carried out on cultured Gilthead sea bream Sparus aurata Linnaeus 1758 from aquaculture sites in Corsica. Factors and biological mechanisms regulating parasites populations, particularly controlling the infestation and survival of monogeneans have been studied. Variations in infection support the hypotheses that the main factor determining the infection of S. chrysophrii was temperature and that the parasite life cycle could be influenced by fish size and fish age. One hundred forty-nine of 280 S. aurata studied were infected. Seasonal differences in prevalence of infection were observed in all farms studied. A similar pattern was observed for parasite abundance with maximal infection levels occurring when water temperature were lower. A statistically significant dependence between infection and host size was also observed. The study of the surface topography of S. chrysophrii with scanning electron microscopy highlighted morphological adaptations not previously described. Parasites have developed diverse mechanisms of survival within the host that facilitate the establishment of infection. We mainly studied sensory structures and the anchoring systems of the parasite, i.e., filaments of eggs and haptor, enabling it to ensure the dispersal and survival of the species.

Among monogeneans that display direct life cycles, plastic developmental strategies may have been selected to counter the lack of transmission opportunities. Within amphibian polystomatids, some species of the genus Polystoma develop into two different phenotypes depending on the host physiological stage to which free swimming larvae attach. When oncomiracidia infest old tadpoles, they develop slowly and migrate during host metamorphosis towards the bladder where they reach maturity. On the other hand when larvae infest young tadpoles, they develop rapidly into neotenic phenotypes that reproduce in the branchial chamber. These alternative developments are explored through experimental infestations with Polystoma gallieni, the specific polystome of the stripeless tree frog Hyla meridionalis. When oncomiracidia were left for 6h in water in which young tadpoles had been previously maintained for one day, they mainly developed into the neotenic phenotype regardless of the tadpole stage they encountered later. This indicates that P. gallieni could collect released host-derived signals before physical contact with its host.

A global study of the Metazoan parasites of Boops boops (L., 1758) have been made in the "golfe du Lion". Fourteen different species (eight Platyhelmintha, one Nematoda and five Crustacea) have been inventorized. Their respective localisation on the hosts, globals and specifics corresponding prevalences as well as their variations according to the size of the fish, and their abundance have been precised. Parasitics associations have also been examined.